Why would cord length make a difference?
Brother just called, he got shocked by a drop cord on a GFCI
- Thread starter hockeyoligist2
- Start date
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Why would cord length make a difference?
- Location
- Illinois
- Occupation
- retired electrician
Give the relative impedance of the person and the cord, I don't see the cord length making any difference.
I don't know why it makes a difference but I have seen gfi's not work on long extention cords several times in the past. I solved this problem by making a short corded recept. box with a gfci recept in it and solved the problem.
Gene
__________________________________________
Remember - Speed Kills and its not always you!
Gene
__________________________________________
Remember - Speed Kills and its not always you!
So you put a box on the end of a cord? I gotta ask......what kind of box?
- Location
- Massachusetts
GFCIs do not have to be grounded to operate correctly.
Where do you connect a ground to a GFCI breaker?
Where do you connect a ground to a GFCI breaker?
Untrue and incorrect. A GFCI does not 'know' the difference between equipment and people solely because a ground is provided. In fact, the only time a GFCI 'knows' there is a ground is when a plug-in tester is used.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
091227-1331 EST
taljacks3:
The mounting strap and its grounding screw connect to none of the electronics in the GFCI, other than by an insignificant amount of capacitance. Therefore, this has nothing to do with the operation of the GFCI. The purpose of the ground termination of the strap is to make sure that the screws holding the faceplate and/or a metal face plate are at ground potential, and provides the EGC connection to the ground pin of the plug.
Ebow:
I see no reason that a GFCI should not work, meaning trip, on the input to a long extension cord from a shunt current of 6 MA from hot to EGC at the end of the cord.
On my test Leviton GFCI it takes between 0.1 and 0.2 mfd from hot to EGC to trip the breaker. The capacitive reactance at 60 Hz is 26539 ohms for 0.1 mfd, and 13269 ohms for an 0.2 mfd. Thus, we have correlation with the expected minimum trip point of a GFCI, 120/.005 = 24000 ohms.
If we assume a 3 conductor extension cord that includes an EGC, and guess at 20 pfd per foot from hot to EGC, then 0.1 mfd corresponds to a cord of 5000 ft. Assuming a reasonable length cord, then the GFCI should not be tripped by the cable capacitance. Thus, the GFCI should be functional.
It is most likely that capacitance from the cable would reduce the trip point and therefore the GFCI would more easily trip on a long cord.
The major problem with a long cord is that you need to walk a long way to reset the GFCI,
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taljacks3:
The mounting strap and its grounding screw connect to none of the electronics in the GFCI, other than by an insignificant amount of capacitance. Therefore, this has nothing to do with the operation of the GFCI. The purpose of the ground termination of the strap is to make sure that the screws holding the faceplate and/or a metal face plate are at ground potential, and provides the EGC connection to the ground pin of the plug.
Ebow:
I see no reason that a GFCI should not work, meaning trip, on the input to a long extension cord from a shunt current of 6 MA from hot to EGC at the end of the cord.
On my test Leviton GFCI it takes between 0.1 and 0.2 mfd from hot to EGC to trip the breaker. The capacitive reactance at 60 Hz is 26539 ohms for 0.1 mfd, and 13269 ohms for an 0.2 mfd. Thus, we have correlation with the expected minimum trip point of a GFCI, 120/.005 = 24000 ohms.
If we assume a 3 conductor extension cord that includes an EGC, and guess at 20 pfd per foot from hot to EGC, then 0.1 mfd corresponds to a cord of 5000 ft. Assuming a reasonable length cord, then the GFCI should not be tripped by the cable capacitance. Thus, the GFCI should be functional.
It is most likely that capacitance from the cable would reduce the trip point and therefore the GFCI would more easily trip on a long cord.
The major problem with a long cord is that you need to walk a long way to reset the GFCI,
.
thats potentially dangerous (besides the possible issue of what box you put it in), and not osha compliant. Portable GFCIs need to have open neutral protection. Cant use a standard GFCI receptacle or deadfront to make a portable GFCI. Have to buy a commercial portable GFCI or use something like a hubbell GFM20
- Location
- Massachusetts
quogueelectric
Senior Member
- Location
- new york
He must have gotten in the path as a load which would not trip the ground fault as it is desighed to detect a fault to ground.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
091227-1715 EST
Ebow:
Define what you mean by a GFCI at the input end of a long extension cord did not work? Did this mean there was no power at the end of the cord? Or if the hot had full voltage and you put a 15,000 resistor between hot and EGC, then it did not trip? What if the 15,000 ohm resistor was from hot to a screw driver in the earth?
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Ebow:
Define what you mean by a GFCI at the input end of a long extension cord did not work? Did this mean there was no power at the end of the cord? Or if the hot had full voltage and you put a 15,000 resistor between hot and EGC, then it did not trip? What if the 15,000 ohm resistor was from hot to a screw driver in the earth?
.
The last case of a GFI not working was on a residental site where the temp. was about 250' away from the house. Three 100' cords run from he temp. into the house, 100' of 10ga, 2 - 100' 12ga. The plumber had and older sawsall with the metal housing laying on the damp concrete floor. kneeling beside a lavatory install he reached for the saw and it bit him. A quick check with the meter showed between 12 and 18 volts to the concrete floor depending on dampness (damp to film of water on floor). Gfi at the temp tested good, each cord tested good individually, but the three strung together would not make the gfi trip. yes it was a faulty sawsall. When grabbed by the plastic handle or the rubber boot it did not shock (naturally), but if you or it were in the damp locations it would shock you when you touched the metal part of the motor housing.
I made a quick gfi j-box with a bell box, a gfi recept., a weather proof cover plate, and a 3 wire pig tail (I carry them on my truck for disposal installs) and plugged it into the end of the cords and the sawsall into it. If someone grabbed the saw by the housing it tripped the gfi at the j-box every time if they were in concact with the damp floor as plumbers usually are.
The plumber quit cussing, except when the gfi tripped, and I went back to work.
Gene
_________________________________
Remember - Speed Kills and its not always you.
I made a quick gfi j-box with a bell box, a gfi recept., a weather proof cover plate, and a 3 wire pig tail (I carry them on my truck for disposal installs) and plugged it into the end of the cords and the sawsall into it. If someone grabbed the saw by the housing it tripped the gfi at the j-box every time if they were in concact with the damp floor as plumbers usually are.
The plumber quit cussing, except when the gfi tripped, and I went back to work.
Gene
_________________________________
Remember - Speed Kills and its not always you.
busman
Senior Member
- Location
- Northern Virginia
- Occupation
- Master Electrician / Electrical Engineer
Customer's are always asking me how a GFCI works. I've been told I have a fairly good layperson explanation. It goes like this:
"The amount of electricity it takes to kill you is about 3 cents. The amount available at the average outlet is about 15 bucks. A circuit works with power going out over one wire and back over the other. Inside the GFCI is a very stingy accountant. As long as the electricity going out and coming back are the same, he leaves the power on. If you try to steal electricity (and get shocked) by touching one wire and SOMETHING ELSE (like the faucet), he notices thefts as small as 1/2 penny and shuts the power off.
Customers seem to like this story.
Happy Holidays,
Mark
"The amount of electricity it takes to kill you is about 3 cents. The amount available at the average outlet is about 15 bucks. A circuit works with power going out over one wire and back over the other. Inside the GFCI is a very stingy accountant. As long as the electricity going out and coming back are the same, he leaves the power on. If you try to steal electricity (and get shocked) by touching one wire and SOMETHING ELSE (like the faucet), he notices thefts as small as 1/2 penny and shuts the power off.
Customers seem to like this story.
Happy Holidays,
Mark
- Location
- Illinois
- Occupation
- retired electrician
I really don't know why the GFCI at the supply end did not trip, but it was not because of the cord length. As a side note, not that makes any difference as to the GFCI tripping, given the fact that the metal housing of the saw was "hot", there had to be an open in the EGC at some point between the supply and the saw.
Remember equipment grounding prevents shocks and GFCIs only limit the duration of the shock.
In the case of the saw, there are differences in the trip times of GFCIs, it could very well be that the only reason that the first one did not trip before the plumber let go of it was a longer trip time and the one from your truck had a shorter trip time.
busman
Senior Member
- Location
- Northern Virginia
- Occupation
- Master Electrician / Electrical Engineer
The trip time on a GFCI should be on the order of 25-30 milliseconds. Could a person really react and let go faster than that?
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
091228-0823 EST
Busman:
See page 4 and Average Trip Time vs Fault Current curve on the datasheet at
http://www.national.com/ds/LM/LM1851.pdf#page=1&search="Ground Fault Interrupter%22
The UL spec maximum is about 6.5 sec at 5 MA from this plot. National's typical time is about 0.4 sec.
Ebow:
My guess is that you did run comparable experiments for your two GFCI locations.
Don gave a good explanation of what may have been your original problem, no complete path of the EGC.
I think the biggest problem with long cables is the increase in the number of random false trips from transients.
Keep in mind that a GFCI does not prevent shocks, but hopefully it reduces the likelihood of death.
.
Busman:
See page 4 and Average Trip Time vs Fault Current curve on the datasheet at
http://www.national.com/ds/LM/LM1851.pdf#page=1&search="Ground Fault Interrupter%22
The UL spec maximum is about 6.5 sec at 5 MA from this plot. National's typical time is about 0.4 sec.
Ebow:
My guess is that you did run comparable experiments for your two GFCI locations.
Don gave a good explanation of what may have been your original problem, no complete path of the EGC.
I think the biggest problem with long cables is the increase in the number of random false trips from transients.
Keep in mind that a GFCI does not prevent shocks, but hopefully it reduces the likelihood of death.
.
busman
Senior Member
- Location
- Northern Virginia
- Occupation
- Master Electrician / Electrical Engineer
Gar,
You are correct. However, the unit you showed says that it is specifically designed to utilize the UL-493 trip time curve and trip as slow as allowed to prevent nuisance trips.
My point was that the standard GFCI duplex or portable plug in GFCI (construction type) isn't this sophisticated and probably trips in less than 100 ms on even a small ground fault.
Thanks for the info.
Mark
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
091228-0911 EST
Busman:
I believe that most standard GFCI receptacles will use the National chip or an equivalent.
A Leviton unit I have opened and circuit traced has an 1851 chip on board.
.
Busman:
I believe that most standard GFCI receptacles will use the National chip or an equivalent.
A Leviton unit I have opened and circuit traced has an 1851 chip on board.
.
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